The invention relates to pneumatic acoustic warning devices used for example an automobile vehicles and boats. The known pneumatic warning devices are constituted by a diaphragm gripped over its entire periphery and pressed by a spring against a circular lip terminating one extremity of a horn. Compressed air admitted on the periphery of the diaphragm external to this lip lifts the diaphragm against the action of the spring and penetrates into the horn, producing vibrations of the diaphragm.
In certain known warning devices the edge of the diaphragm is gripped against the periphery of a body by a screwed lid and the body comprises the pressing lip of the diaphragm and a support for the horn and a foot. The body and the lid are made by moulding from a zinc alloy which must then be subjected to surface treatment. The assembly is heavy and not very aesthetic, by reason of the moulding requirements. Furthermore it is troublesome since it necessitates a mould for each warning device model and utilises a large quantity of costly alloy. Finally it necessitates very strict inspection since any moulding defect necessitates rejection of the apparatus since any burr on the lip would involve a "wound" to the diaphragm.